Usually, high carbon steel wires each having a very small diameter have been hitherto produced by way of the steps of allowing a steel material to be subjected to the rolling as desired, subsequently, controllably cooling the hot-rolled steel rod, allowing the cooled steel rod to be subjected to primary drawing to prepare a steel wire having a diameter of 5.0 to 5.5 mm, allowing the steel wire to be subjected to final patenting treatment, and thereafter, plating the steel wire with a brass, and finally, allowing it to be subjected to final drawing in a wet state. Many steel wires of the aforementioned type each having a very small diameter have been practically used in the form of a steel cord produced such that it is made of strands or bunches. As desired, a wire stranding or bunching operation is optionally performed to produce a steel cord having two steel wires stranded together, having seven steel wires stranded together or the like. To this end, it is necessary that each steel wire has excellent ductility sufficient to resist a severe wire stranding or bunching operation performed at high speed (in excess of 18000 rpm).
In addition, each steel wire is required to have high tensile strength, sufficient toughness and excellent resistibility against fatigue breakage. To satisfactorily meet the foregoing requirement, a variety of development works have been heretofore conducted to produce a steel material having a high quality.
For example, steel wires each having a very small diameter and sufficient toughness and high carbon steel wires employable as a steel cord, both of which are produced with low occurrence of wire breakage during a stranding operation by restrictively defining a content of manganese less than 0.3% to suppress the generation of an excessively cooled structure after completion of a lead patenting treatment, and moreover, restrictively defining the content of each of C, Si, Mn and other elements, are disclosed in an official gazette of Japanese Unexamined Publication Patent (Kokai) No. 60-204865. In addition, a steel rod usable for producing steel wires each having a very small diameter, sufficient toughness and excellent ductility, which are produced at a reduced drawing rate using steel rods each of which is subjected to a lead patenting treatment to elevate tensile strength with a content of silicon set to 1.00% or more, are disclosed in an official gazette of Japanese Unexamined Publication Patent (Kokai) No. 63-24046. Additionally, a steel rod having elements of Al, Ti, Nb and Zr added thereto by a quantity of 0.01% or more to improve ductility of the steel rod in the presence of a carbide and a nitride, wherein the maximum width of a segregation zone where carbon or manganese is segregated by a quantity as much as 1.3 times the average content of carbon or manganese within the range of less than a half of the radius of the steel rod as measured form the center of a cross-sectional plane of the steel rod determined to be 0.01 or less of a diameter of the steel rod are disclosed in an official gazette of Japanese Unexamined Publication Patent (Kokai) No. 62-238327.
The prior invention disclosed in the official gazette of Japanese Unexamined Publication Patent (Kokai) No. 60-204865 is concerned with a high carbon steel rod employable in producing steel wires each having a very small diameter of 0.5 mm or less and a tensile strength of 250 kgf/mm.sup.2 or more by way of a step of wire drawing, and the prior invention disclosed in the official gazette of Japanese Unexamined Publication Patent (Kokai) No. 63-14046 is concerned with a high carbon steel rod employable in producing steel wires each having a very small diameter of 0.5 mm or less and a tensile strength of 300 kgf/mm.sup.2 or more.
In recent years, however, earnest requests for increasing tensile strength of each steel wire for producing steel cords have been forthcoming from users in proportion to the latest accelerated reduction of the weight of each tire and increased performance of the same. To satisfy the foregoing requests, a variety of development works have been hitherto conducted to produce steel cords each having a tensile strength having an order of 340 kgf/mm.sup.2. In addition, it is expected by users that steel codes each having a tensile strength of 360 kgf/mm.sup.2 or more will be practically produced on an industrial basis.